1. Field of the Invention
This invention relates to a semiconductor laser device with a double-heterostructure.
2. Description of the Prior Art
Conventional index-guided semiconductor laser devices with a striped structure include buried-type laser devices that have a low threshold current level. Typical examples of the buried-type laser devices are BH (buried heterostructure) lasers disclosed by, for example, Japanese Patent Publication Nos. 52-40958, 52-41107 and 52-48066 and DC-PBH (double-channel planar buried heterostructure) lasers disclosed by, for example, Japanese Patent Publication Nos. 62-2718 and 62-7719.
FIGS. 6A and 6B, respectively, show a BH laser and an InGaAsP/InP DC-PBH laser in which on a semiconductor substrate 61, a mesa stripe constituted by a cladding layer 62, an active layer 63 and a cladding layer 64 is disposed, and a burying layer 60 is disposed outside of the mesa. The reference numerals 68 and 69 indicate electrodes.
The above-mentioned buried-type laser devices with an index waveguide that is formed within the active layer 63 for laser oscillation are advantageous in that they oscillate laser light according to an index waveguiding operation and have a low threshold current of about 20 mA or less.
However, if a proper refractive index is not applied to the burying layer 60 made of a low refractive index semiconductor material, and if a proper width W is not applied to the waveguide that corresponds to the mesa, the laser devices will oscillate in a high-order transverse mode. As a result, laser light emitted from the device cannot be concentrated into a spot by any optical lens, which makes the practical use of the device inconvenient. To eliminate such a high-order transverse mode, it is necessary to narrow the mesa width W so as to be as small as 1-2 .mu.m, which causes the laser-emitting face to break down even at a relatively low optical output power, and which causes difficulties in the production of the device, so that mass-production of the device cannot be attained and reliability of the device is decreased.
On the other hand, with the index guided laser devices, there are CSP (channeled substrate planar) laser devices that are disclosed by, for example, Japanese Patent Publication 54-5273. FIG. 7 shows a CSP laser device in which on an n-GaAs substrate 71 with a striped channel 77 that is formed into a rectangularity in section, an n-GaAlAs cladding layer 72, a GaAlAs active layer 73, a p-GaAlAs cladding layer 74, and an n-GaAs cap layer 75 are disposed in that order. A p-Zn diffusion layer 76 is disposed in the area from the cap layer 75 to the cladding layer 74 corresponding to the rectangular channel 77. The rectangular channel including both the shoulder portions S defines the borderline between the substrate 71 end the cladding layer 72. The thickness of the portions of the cladding layer 72 that are outside of the rectangular channel 77 is so thin that the laser light L produced in the active layer 73 can permeate into a absorption layer (i.e., the GaAs substrate 71), whereas the thickness of the portion that corresponds to the rectangular channel 77 is so thick that the laser light L cannot be absorbed into the absorption layer 71.
Thus, in the above-mentioned CSP laser device, the effective refractive index of the portion of the active layer 73 corresponding to the rectangular channel 77 becomes smaller than that of the portions of the active layer 73 corresponding to the outside of the rectangular channel 77, resulting in an index waveguide within the active layer 73. This laser device is also advantageous in that it tends to oscillate in a fundamental transverse mode because high-order transverse mode gain is suppressed by the phenomenon that the laser light from the portions of the active layers outside of the rectangular channels 77 is absorbed by the absorption layer 71 as mentioned above.
However, if the thickness of the portions of the cladding layer 72 corresponding to the outside of the rectangular channel 77 is thin in excess, the absorption of laser light in these areas will arise exceedingly, resulting in the emission of laser light with a low differential quantum efficiency. If the thickness of the cladding layer 72 corresponding to the outside of the rectangular channel 77 is thick in excess, the effective refractive-index difference .DELTA.n of the optical waveguide will become small, which makes the transverse mode unstable. That is, the CSP laser device attains laser oscillation in a fundamental transverse mode, but the oscillation spot obtained shifts with an increase in an optical output power (i.e., with an increase in current to be injected). When the oscillation spot significantly shifts, as shown in FIG. 8, a kink K that is named an I-L kink occurs in the current/optical output characteristic curve. This phenomenon is explained below: Since the laser oscillation spot LS permeates into the substrate 71 outside of the rectangular channel 77 as shown in FIG. 7, the differential quantum efficiency.eta..sub.d of the laser light varies depending upon the thickness of the cladding layer 72 outside of the rectangular channel 77; namely, when the thickness thereof is thin in excess, the differential quantum efficiency .eta.d becomes exceedingly small and when the thickness thereof is thick in excess so as to make the .eta.d great, the effective refractive-index difference .DELTA.n of the optical waveguide becomes small so that the oscillation spot will shift at a certain optical output power and the kink K in the current/optical output power characteristic curve such as that shown in FIG. 8 will occur.
Accordingly, in order for the CSP laser device to attain oscillation in a stable fundamental transverse mode and to attain a satisfactory differential quantum efficiency, the thickness of the portions of the cladding layer 72 outside of the rectangular channel 77 must be precisely regulated. Although the CSP laser device with the above-mentioned rectangular channel having the shoulder portions S is produced by LPE (liquid phase epitaxy), LPE does not provide layers with a predetermined uniform thickness, so that the resulting CSP laser device has a low differential quantum efficiency and oscillates in an unstable transverse mode.